1. Technical Field
The invention is related to plasma reactors for processing semiconductor wafers, and in particular confinement of the processing plasma in the reactor within a limited processing zone.
2. Background Art
Plasma reactors, particularly radio frequency (RF) plasma reactors of the type employed in semiconductor wafer plasma processing in the manufacturing of microelectronic integrated circuits, confine a plasma over a semiconductor wafer in the processing chamber by walls defining a processing chamber. Such an approach for plasma confinement has several inherent problems where employed in plasma reactors for processing semiconductor wafers.
First, the walls confining the plasma are subject to attack from ions in the plasma, typically, for example, by ion bombardment. Such attack can consume the material in the walls or introduce incompatible material from the chamber walls into the plasma process carried out on the wafer, thereby contaminating the process. Such incompatible material may be either the material of the chamber wall itself or may be material (e.g., polymer) previously deposited on the chamber walls during plasma processing, which can flake off or be sputtered off. As one example, if the chamber walls are aluminum and the plasma process to be performed is plasma etching of silicon dioxide, then the material of the chamber wall itself, if sputtered into the plasma, is incompatible with the process and can destroy the integrity of the process.
Second, it is necessary to provide certain openings in the chamber walls and, unfortunately, plasma tends to leak or flow from the chamber through these openings. Such leakage can reduce plasma density near the openings, thereby upsetting the plasma process carried out on the wafer. Also, such leakage can permit the plasma to attack surfaces outside of the chamber interior. As one example of an opening through which plasma can leak from the chamber, a wafer slit valve is conventionally provided in the chamber side wall for inserting the wafer into the chamber and withdrawing the wafer from the chamber. The slit valve must be unobstructed to permit efficient wafer ingress and egress. As another example, a pumping annulus is typically provided, the pumping annulus being an annular volume below the wafer pedestal coupled to a vacuum pump for maintaining a desired chamber pressure. The chamber is coupled to the pumping annulus through a gap between the wafer pedestal periphery and the chamber side wall. The flow of plasma into the pumping annulus permits the plasma to attack the interior surfaces or walls of the pumping annulus. This flow must be unobstructed in order for the vacuum pump to efficiently control the chamber pressure, and therefore the pedestal-to-side wall gap must be free of obstructions.
It is an object of the invention to confine the plasma within the chamber without relying entirely on the chamber walls and in fact to confine the plasma in areas where the chamber walls to not confine the plasma. It is a related object of the invention to prevent plasma from leaking or flowing through openings necessarily provided the chamber walls. It is an auxiliary object to so prevent such plasma leakage without perturbing the plasma processing of the semiconductor wafer.
It is a general object of the invention to shield selected surfaces of the reactor chamber interior from the plasma.
It is a specific object of one embodiment of the invention to shield the interior surface of the reactor pumping annulus from the plasma by preventing plasma from flowing through the gap between the wafer pedestal and the chamber side wall without obstructing free flow of charge-neutral gas through the gap.
It is a specific object of another embodiment of the invention to prevent plasma from flowing through the wafer slit valve in the chamber side wall without obstructing the ingress and egress of the wafer through the wafer slit valve.
The invention is embodied in a plasma reactor including a chamber enclosure having a process gas inlet and including a ceiling, a sidewall and a workpiece support pedestal capable of supporting a workpiece at a plasma processing location facing the ceiling, the workpiece processing location and ceiling defining a process region therebetween, the pedestal being spaced from said sidewall to define a pumping annulus therebetween having inner and outer walls, to permit process gas to be evacuated therethrough from the process region. The invention further includes a pair of opposing plasma confinement magnetic poles arranged adjacent the annulus within one of the inner and outer walls of the annulus, the opposing magnetic poles being axially displaced from one another the opposite poles being oriented to provide maximum magnetic flux in a direction across the annulus and a magnetic flux at the processing location less than the magnetic flux across the annulus.